Loudspeakers



May 25, 1965 Filed Oct. 10, 1960 H. F- OLSON ET AL v LOUDSPEAKERS 5 Sheets-Sheet l INVENTORS BY JEAN Pris-rod May 25, 1965 H. F. OLSON ETAL LOUDSPEAKERS 3 Sheets-Sheet 2 Filed Oct. 10, 1960 May 25, 1965 H. F. OLSON ET'AL LOUDSPEAKERS 3 Sheets-Sheet 3 Filed 061,- 10, 1960 y Jay/v PRESTON United States Patent corporation of Delaware Filed Oct. 10, 1960, Ser. No.61,537

9 Claims. (Cl. 179-1) The present invention relates to sound reproducing devices such as loudspeakers and more particularly to loudspeakers which synthesize sound reverberation envelopes.

A listener in a concert hall hears a sound pattern composed of two separate acoustical effects, namely the sounds which come directly from the performers and the multiple reflections of these sounds from the surfaces of the concert hall. The direct sound enables the listener through his binaural sense to determine the auditory perspective, i.e. the spatial localization of the original sources of sound. The multiplicityof reflected sounds :from the surfaces of the hall provide the listener with a reverberation envelope. These two acoustical effects constitute the artistic aspects of concert hall renditions.

The phenomenon of reverberation is so common in everyday life and audiences have been so long accustomed to hearing performances in auditoriums with considerable reverberation, that when reverberation is absent, the sound heard appears unnatural and lacking in expected depth. The lack of space in most recording studios makes it difficult to make recordings with proper reverberation.

Many systems heretofore proposed for synthetically adding reverberation to sound have been both exceed ingly complex and expensive. Attempts to add reverberation to loudspeakers which also produce direct sound have resulted in insufiicient reverberation envelopes being developed and an absence of concert hall realism in the home.

Accordingly, it is an object of this invention to provide a new and improved loudspeaker for the production of reverberant sound.

It is another object of this invention to provide a new and improved loudspeaker reverberator which provides only reverberant sound.

It is a further object of this invention to provide a loudspeaker which synthesizes reverberation by the use of simple, inexpensive mechanical components.

In accordance with the invention, an electro-mecha-nical transducer is coupled to a mechanoaacoustical transducer by a mechanical transmission line having both time delay vibration-transmitting and vibration-reflecting characteristics. More specifically a dynamic driver such as that of a loudspeaker is coupled to a sound radiating cone or diaphragm by means of one or more coiled springs. Electrical impulses transduced into mechanical vibrations by the dynamic driver are transmitted by the springs to the diaphragm, the piston-like movement of which causes sound to be radiated therefrom. The energy not absorbed by the diaphragm is reflected back through the springs to the dynamic driver. The energy is again reflected by the dynamic driver and then retransmitted by the springs. Thesuccessive transmissions and reflections of the energy synthetically produce a reverberation envelope.

The novel features which are considered to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to organization and method of operation, as Well as additional objects and advantages thereof, will best be understood by referring to the accompanying drawing and the following description in which:

FIGURE 1 is a sectional view of a loudspeaker em- 3,185,767 Patented May 25, 1965 ice bodying the invention, with the helical spring shown in full view for clarity;

FIGURE 2 is a sectional view of a loudspeaker incorporating the preferred embodiment of the invention;

FIGURE 3 is a view taken along the section lines 3-3 in FIGURE 2; p 7

FIGURE 4 is a graph diagrammatically illustrating the reverberation characteristics of the embodiment of the invention shown in FIGURE 2;

FIGURES 5 and 6 are partly broken sectional views of other loudspeakers incorporating the invention; and,

FIGURE 7 is a block diagram of a sound reproducing system including a loudspeaker reverberator constructed in accordance with the invention.

Referring to the drawing, wherein like reference numerals have been given to like components in the various figures thereof, and particularly to FiGURE l, a loudspeaker 10 constructed in accordance with the invention includes an electro-mechanical transducer coupledto a mechano-acoustical transducer by means of a mechanical transmission line.

More specifically, a permanent magnet 16 is mounted on a metallic housing 18 which includes an inwardly extending annular top pole plate 20. A voice coil 22 is suspended in an air gap formed between the magnet 16 and the central aperture in the pole plate 20 by means of a disc 24 and a pair of compliant annular centering supports or spiders 26 and 28. The centering supports 26 and 28 are cemented to both the annular pole plate 20 and the disc 24 and serve to maintain the voice coil 22 properly centered in the air gap with little or no transverse movement possible but capable of extreme axial motion. The aforementioned components comprise the electromechanical transducer or dynamic driver ot the loudspeaker 10.

A spring 30, having a multiple number of coils or turns wound in the form of a helix with the diameter of the coils being constant throughout the length of the spring, is provided to comprise the mechanical transmission line in accordance with the invention. The ends of the spring 30 are soldered to a pair of end plates 32 and 34 respectively. The end plate 32 is suitably fastened to the disc 24 while the other end plate 34 is fastened to a hub 36.

A conical truncated diaphragm 38 is provided to comprise the mechano-acoustical transducer in accordance with the invention. The apex of the conical diaphragm 38 is cemented to the hub 36 and the outer peripheral rim of the base of thediaphragrn 38 is clamped between and supported by a pair of annular rings 4% and 42.

A supporting structure for the loudspeaker 10 includes a frame member 44 and a plurality of threaded rods 46. The frame member 44 is fastened to the pole plate 20. The rods 46 are inserted through a plurality of apertures on the frame member 44 and the annular rings 40 and 42 and then fastened to a suitable cabinet 48 which holds and encloses the loudspeaker 10. A plurality of nuts 49 are provided to secure the rods 46 to the cabinet 48, and the frame member 44 and the annular rings 40 and 42 to the rods 46.

When electrical impulses are applied to the voice coil 22, the voice coil 22 is sent into vibratory motion. This is due to the force exerted thereon by an electrical current flowing, in the magnetic field created between permanent magnet 16 and pole plate 20. The oscillatory motor vibration of the voice coil 22 is applied to the spring 3%) and mechanical vibrations are transmitted down the length of the spring 30 by the axial displacement of successive coils. After a predetermined time delay, the mechanical vibratory energy arrives at the diaphragm 38. A part of the energy in the spring 30 is transmitted answer Q! to the diaphragm 38 causing piston-like vibrations to be set up therein which radiates the energy in the form of sound Waves.

Substantially all the mechanical vibratory energy that is not absorbed by the diaphragm 38 is reflected back through the spring 30. The reflected wave travels back to the voice coil where it is substantially completely reflected again. The Wave then is again transmitted toward the diaphragm 3S and the process repeats itself.

Successive transmissions, reflections and retransmissions of energy occur with decreasing amplitude in each cycle until the acoustical energy radiated is reduced to the point of inaudibility. Thus the loudspeaker synthesizes the multiple reflections of sound in a reverberant auditorium Where acoustical energy impinging on one surface is partially absorbed and partially reflected. The reflected sound again impinges on a second surface, etc., the process repeating until inaudibility is reached.

No sound is directly radiated from the dynamic driver of loudspeaker 10. All energy radiated in the form of sound waves occurs after the introduction of a time delay by the mechanical transmission line or spring 30. Energy received by the voice coil 22 in the form of electrical impulses is stored in the spring during the time it takes to be transmitted along the spring to the diaphragm 38. The diaphragm 33 radiates sound waves only after the spring 30 transmits energy thereto. Therefore, no sound emanates from the loudspeaker 10 during the storage time period.

Any form of transmission line that introduces a time delay in the transmission of wave energy and also causes successive and diminishing reflections of the Wave energy may be used in the invention. However a mechanical transmission line in the form of a spring is very practical. The velocity of longitudinal wave propagation in a spring 35 s relatively small, making the transit time of the Wave energy relatively long. Thus the desired comparatively long time delay between the initial vibration of the dynamic driver and the radiation of sound from the loudspeaker is attained. Furthermore the dissipation of energy in a spring is relatively small providing a substantially lossless transmission line. A spring transmission line is also easily terminated in an impedance much smaller than its characteristic impedance to cause the desired reflection of wave energy. Therefore a spring is well suited as for providing the delay and reflection necessary for synthesizing reverberation in a sound reproducing device.

The velocity of Wave propagation in a helical spring is given by the equation,

ii H

where V=velocity in centimeters per second m=mass of the spring per unit length C=comp1iance of the spring per unit length.

In terms of the constants of the spring the mass and compliance per unit length are: 0

where =density of the material of the spring in grams per cubic centimeter E shear modulus in dynes per cubic centimeter R radius of the coils or turns in centimeters r radius of the wire in centimeters n=total number of turns l=length of spring in centimeter For steel wire, the shear modulus and density are 8X10 dynes per cubic centimeter and 8 grams per cubic centimeter respectively and Equation 4 for steel reduces to Steel springs designed to provide transit times of from 20 to 30 milliseconds have provided proper time delays.

The characteristic mechanical impedance of a helical spring may be determined from the equation where the terms in the equation have been defined above. As long as the spring transmission line is not terminated in a resistance having a magnitude equal to the characteristic impedance, reflection of vibratory wave energy Will occur. However it is important that there be a large mismatch between the characteristic mechanical impedance of the spring 30 and the terminating resistance, so that a major portion of the vibratory wave energy is reflected. A simple introduction of a single time delay by the spring 3% would not produce reverberation but only a single echo.

The terminating impedance in the loudspeaker 10 is substantially equal to the mechanical impedance of the diaphragm 38 and the mechanical radiation resistance of the air load on the diaphragm 38. The mechanical radiation resistance is mainly determined by the dimensions of the diaphragm 38. A diaphragm 38 is chosen so that the radiation resistance is very small compared to the characteristic impedance of the spring 30. For example, at 500 cycles the mechanical radiation resistance is that of the characteristic impedance of the spring. As a consequence, the sound waves in the spring must make a large number of excursions in order to transfer the major or significant amount of energy stored in the spring to radiated sound energy. Thus, it will be seen that the radiated sound from the loudspeaker 10 simulates the reflected sound in an auditorium, namely, the first element of radiated sound is delayed with respect to the electrical impulses in the dynamic driver and the elements of sound which are radiated persist for a time after the electrical impulses have ceased. In other words, the output of the loudspeaker 10 possesses all the characteristics of reverberation.

Reverberation time, which is a measurement of acoustical quality, is arbitrarily defined as the time required for sound intensity to decrease to one-millionth of the original intensity. Any desired reverberation time may he obtained in a loudspeaker constructed in accordance with the invention.

Assuming a sine wave electrical signal input to the dynamic driver of the loudspeaker 10, the sound pressure amplitude at any time after the input signal has been removed is given by the equation mt (7) P=P e 2M sin 21rft where P=sound pressure in dynes per square centimeter at any time t t=time in seconds R =effect1ve mechanical resistance of the mechanical system III mechanical ohms M=effective mass of the vibrating system, in grams P =souud pressure at t=i in dynes per square centimeter j=frequency in cycles per second.

The term use 6 2M determines the exponential decay of the sound emanating from the loudspeaker 10 and therefore the reverberation time.

The time required for the radiated sound intensity to decay to one millionth after the electrical impulses have ceased is the reverberation time. Since sound intensity is propoitional to the square of the sound pressure, the

reverberation time then is given by the expression Rmt effective mass of the vibrating system is the mass of cone 38, the end plates 24 and 36, the voice coil 22, and the a spring in which a wave system exists is /2 of the mass of the spring. For one embodiment of FIGURE 1 in which the cone diameter was 6 inches, the total effective mass of the vibrating system as outlined above was 420 grams. For the above constants of effective mechanical resistance and effective mass of the vibrating system, the reverberation time as computed from the above equation is four seconds at 500 cycles per second. Reverberation times of from 0.5 to seconds provide excellent simulation of concert hall acoustical effects and are attained in a loudspeaker constructed in accordance with the invention.

Referring to FiGURE 2, a preferred embodiment of the invention includes a plurality of springs 52, 54, 56 and 58 connected in a loudspeaker 50 between the disc 24 and the hub 36. FIGURE 3 shows the arrangement of the springs with respect to each other when mounted and the difference in coil or turn diameter in each spring.

The inclusion of a multiple number of springs with different diameters introduces different wave propagation velocities into the wave energy transmitted by the springs. The more springs that are included in the loudspeaker 50, the more closely would the sound emanating from the loudspeaker 50, simulate the multiple reverberations in a concert hall, where the tremendously large number of reflected sound waves arriving at a listener over different paths having different transit times creates an envelope of reverberant sound.

The loudspeaker 50 shown in FIGURE 2 was constructed using steel wire as the material for the springs. The diameter of the wire was selected to be 0.1 inch and the coil or turn radii in each of the four springs 52, 54, 56 and 58 were inch, inch, 1 inch and 1% inches respectively. The total number of coils or turns in each spring was 60. The dynamic driver and diaphragm of a standard 8 inch loudspeaker were also used. The loudspeaker 50 was mounted in a cabinet of one cubic foot volume.

The reverberation time versus frequency characteristic of the loudspeaker 50 is graphically illustrated in FIG- URE 4. While presumably a very large number of springs would have to be included in the construction of the effective mass of the spring 30. The effective mass of 7 loudspeaker St} in order to simulate exactly the acoustics of a concert hall, the reverberation time measured for this loudspeaker was about the optimum in the mid-frequency range, centered about 1000 cycles. In a concert hall, the reverberation time is of the order of 1.5 seconds with the optimum time depending on the volume and geometrical configuration of the hall.

Referring to FIGURES 5 and 6, loudspeakers embodying the invention are shown which use only a single spring rather than a multiple number of springs. By varying the coil or turn diameter along the length of a spring, continuous Wave energy reflection characteristics will be created in the spring itself by the mismatch of mechanical impedance between successive coils. Consequently a large number of reflections of wave energy with varying time delays occur because of the introduction of a succession of impedance discontinuities along the mechanical transmission line or spring.

In FIGURE 5, a spring 60 wound in the form of a helix having an hourglass shape is utilized, while in FIGURE 6 a spring 62 wound in the form of a t'runcated'helical cone is utilized to accomplish the continuous reflections of vibratory wave energy.

As shown in FIGURE 7, a system for simulating concert hall realism in the home includes a stereophonic sound reproducer with the addition of a loudspeaker reverberator constructed in accordance with the invention. A stereophonic phonograph 70 includes a motor 71, a record turntable 72 suitably mounted on the motor 71 to be driven thereby, and a sound signal pick-up device or stylus 73 mounted to track the grooves in a stereophonic disc record 74. The pick-up device 73 is connected to left 75 and right 76 variable audio frequency amplifiers, the amplified outputs of which are fed to left 77 and right 78 loudspeakers respectively. The loudspeakers 77 and 78 are known direct-radiator conicaldiaphragm type loudspeaker units and are preferably spaced laterally apart from each other. i

The amplified sound signals from the amplifiers 75 and 76 are also mixed or combined in a mixer and amplifier stage 79 before application to a loudspeaker reverberator 80 which is constructed in accordance with the invention. The loudspeaker reverberator 80 is preferably located to the rear of a listener.

In listening to sound being reproduced by the above system, a greatly enhanced concert hall realism may be attained using ordinary stereophonic disc records. The direct sound left and right loudspeakers 77 and 78 provide the auditory perspective and the time delayed and reflected sound from the loudspeaker reverberator 80 provides the reverberation envelope. No direct sound emanates from the loudspeaker reverberator 80. The only direct sound that is heard comes from the loudspeakers 77 and 78, with the reverberator loudspeaker 80 supplying the reverberation after a predetermined time delay, which is the normal sequence in auditorium acoustical effects.

The ratio of direct sound to reverberant sound is also controlled in this system by varying the volume controls on the amplifiers 7'5, 76 and 79. Thus, controlled reverberant sound may be added to a st-ereophonic sound reproducing system in the home by adding a loud-speaker constructed in accordance with the invention, thereby achieving substantially all the acoustical effects experienced in a concert hall.

What is claimed is:

1. A loudspeaker reverberator comprising in combination an electro-mechanical transducer; a spring wound in the form of a helix and having a given characteristic mechanical impedance, coupled at one end :to said electromechanical transducer; said spring having :a time delay vibration-transmitting characteristic; and a mechanoacoustical transducer coupled to the other end of said spring, said mechano-acoustical transducer having a lesser mechanical resistance than the characteristic impedance of said spring whereby vibration reflection occurs at said mechano-acoustical transducer.

2. A loudspeaker reverberator comprising in combination a dynamic driver for converting electrical impulses into mechanical vibrations, a helical spring having a given characteristic impedance and a transit time for mechanical vibrations of from 20 to 30 milliseconds coupled at one end to said dynamic driver, and a diaphragm having an air load thereon exhibiting a mechanical radiation resistance less than the characteristic impedance of said spring coupled to the other end of said spring, said diaphragm reflecting a major portion of the mechanical vibrations transmitted thereto and radiating the rem-aining portion in the form of sound waves.

3. A loudspeaker reverberator comprising in combination an elect-ro-mechanical transducer to translate received electrical impulses into mechanical vibrations, a helical spring having a given characteristic impedance and a given time delay vibration transmitting characteristic coupled at one end to said electromechanical transducer to transmit said vibrations, and a mechanoacoustical transducer coupled to the other end of said spring to receive the mechanical vibrations, said mechano-acoustical transducer having a mechanical resistance less than the characteristic impedance of said spring in order to reflect a major portion of the mechanical vibrations and translate the remaining portion into sound waves, said loudspeaker providing a reverberation time of from 0.5 to 5 seconds.

4. A loudspeaker having reverberation synthesizing characteristics comprising in combination a dynamic driver for converting electrical impulses into mechanical vibrations, a plurality of reverberator springs Wound in the form of helixes, each of said springs having a different diameter, means coupling said dynamic driver to one end of each spring for imparting to said springs 'mechanical vibrations, each of said springs having a different time delay vibration-transmitting characteristic; a diaphragm for translating mechanical vibrations into sound waves; means coupling said diaphragm to the other ends of each of said springs to receive mechanical vibrations having different time delays, said diaphragm reflecting a portion of the mechanical vibrations back to said springs.

5. A loudspeaker having reverberation synthesizing characteristics comprising in combination a dynamic driver for converting electrical impulses into mechanical vibrations, a spring having a given characteristic mechanical impedance coupled at one end to said dynamic driver for receiving and transmitting the mechanical vibrations, said spring comprised of a plurality of helical coils of different diameters for continuously reflecting a portion of the mechanical vibrations at each successive coil, a mechano-acoustical transducer coupled to the other end of said spring to receive mechanical vibrations therefrom after a predetermined time delay measured from the initial vibration of said dynamic driver, said mechano acoustical transducer translating a portion of said mechanical vibrations into radiated sound Waves and reflecting the remaining portion.

6. A loudspeaker as defined in claim 5 wherein said spring is wound in the form of a helix having an hourglass shape.

7. A loudspeaker as defined in claim 5 wherein said spring is wound in the form of a helix having a truncated cone shape.

8. A loudspeaker reverberator comprising in combination a voice coil having a given mass suspended in a magnetic field for translating electrical impulses into mechanical vibrations, a helical spring having a given mass for transmitting the mechanical vibrations, a diaphragm having a given diameter to obtain a predetermined mechanical radiation resistance and a given mass, said diaphragm translating the mechanical vibrations into acoustical vibrations, and first and second end plates each having a given mass for coupling said voice coil to said spring and said spring to said diaphragm respectively, said mechanical radiation resistance bearing a predetermined relationship to the effective masses of said spring, voice coil and end plates so that said loudspeaker provides a predetermined reverberation time.

9. A sound reproducing system for simulating concert hall acoustical etltects comprising in combination a pair of direct sound radiating loudspeakers; means for applying to said loudspeakers amplified stereophonicallyrelated sound signals; a reverberator loudspeaker comprising an elect ro-mechanical transducer, a mechanoacoustical transducer and a mechanical transmission line coupling together said transducers, said mechanical transmission line having a time delay vibration-transmitting and vibration-reflecting characteristic, and means :for applying to said reverberator loudspeaker a mixed composite sound signal derived from said stereophonicallyrelated signals.

Reterences Cited by the Examiner UNITED STATES PATENTS 1,932,461 6/32 Van Deventer 179l09 2,230,836 2/4l Hammond 179--l 2,375,004 5/43 Knowles 35-25 2,548,235 4/51 Olson 1'791 2,768,235 10/56 Knoblaugh 1'791 2,853,145 9/58 Martin 179-1 ROBERT H. ROSE, Primary Examiner.

L. MILLER ANDRUS, WILLIAM C. COOPER,

Examiners. 

1. A LOUDSPEAKER REVERBERATOR COMPRISING IN COMBINATION AN ELECTRO-MECHANICAL TRANSDUCER; A SPRING WOUND IN THE FORM OF A HELIX AND HAVING A GIVEN CHARACTERISTIC MECHANICAL IMPEDANCE, COUPLED AT ONE END TO SAID ELECTROMECHANICAL TRANSDUCER; SAID SPRING HAVING A TIME DELAY VIBRATION-TRANSMITTING CHARACTERISTIC; AND A MECHANOACOUSTICAL TRANSDUCER COUPLED TO THE OTHER END OF SAID SPRING, SAID MECHANO-ACOUSTICAL TRANSDUCER HAVING A LESSER MECHANICAL RESISTANCE THAN THE CHARACTERISTIC IMPEDANCE OF SAID SPRING WHEREBY VIBRATION REFLECTION OCCURS AT SAID MECHANO-ACOUSTICAL TRANSDSUCER. 